Temperature Dependence of the Fluorescence Properties of Curcumin

ARTICLE pubs.acs.org/JPCA

Temperature Dependence of the Fluorescence Properties of Curcumin Yuval Erez, Itay Presiado, Rinat Gepshtein, and Dan Huppert* Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel

bS Supporting Information ABSTRACT: Steady-state and time-resolved techniques were employed to study the nonradiative process of curcumin dissolved in ethanol and 1-propanol in a wide range of temperatures. We found that the nonradiative rate constants at temperatures between 175250 K qualitatively follow the same trend as the dielectric relaxation times of both neat solvents. We attribute the nonradiative process to solvent-controlled proton transfer. We also found a kinetic isotope effect on the nonradiative process rate constant of ∼2. We propose a model in which the excited-state proton transfer breaks the planar hexagonal structure of the ketoenol center of the molecule. This, in turn, enhances the nonradiative process driven by the twist angle between the two phenol moieties.

’ INTRODUCTION Curcumin (shown in Scheme 1) is the main curcuminoid of the Indian spice and food coloring agent turmeric, which is a member of the ginger family.1,2 Curcumin can exist in at least two tautomeric forms in the electronic ground-state: keto and enol. The enol is the more stable form in both the solid phase and solution.3,4 In numerous studies over the last two decades, it was found that curcumin has antitumor, antioxidant, antiarthritic, antiamyloid, anti-ischemic, and anti-inflammatory properties, and it is currently under clinical trials in humans.57 Curcumin exhibits interesting photophysical and photochemical properties.1,813 The optical absorption maximum of curcumin depends on the properties of the solvent and ranges between ∼408430 nm in a wide range of nonpolar, polar, and protic organic solvents, whereas the emission maximum in these liquids is even more sensitive to the properties of the solvent and lies in the range of 460560 nm. Thus, the Stokes shift varies in different solvents from 2000 to 6000 cm1.9 The fluorescence quantum yield in most of the solvents is low and is reduced significantly in the presence of water. The fluorescence lifetime in liquids at room temperature is short ( 1 IðνÞ ¼ h 0 α e 1 ð1Þ α 2γðν νp Þ=Δ

Figure 2. Fluorescence up-conversion signals of curcumin in (a) ethanol and (b) 1-propanol.

ð2Þ

Here, νp, h, Δ, and γ are the peak position, amplitude, width, and asymmetry, respectively. When γ = 0, the line-shape is Gaussian. The values of the analysis parameters are given in Table S1 in the Supporting Information (SI). At 84 K, the OO transition is positioned at 20 920 cm1. The bandwidth is 1250 cm1, and it increases to about 2000 cm1 at room temperature. A similar temperature dependence of the steadystate emission was found for curcumin in 1-propanol. These results are given in the SI. Figure 1b shows the steady-state emission spectra of curcumin in ethanol and their fits at low, intermediate, and high

Figure 3. Time-resolved spectra of curcumin in (A) ethanol and (B) 1-propanol at several times constructed from the fluorescence up-conversion data shown in Figure 2. 10964

dx.doi.org/10.1021/jp206176p |J. Phys. Chem. A 2011, 115, 10962–10971

The Journal of Physical Chemistry A

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Figure 4. Time-resolved TCSPC emission signals of curcumin in ethanol at several temperatures on a semilogarithmic scale.

Figure 5. Time-resolved emission of curcumin in ethanol at 222 K, measured at several wavelengths in the spectral range 470610 nm.

Table 1. Average Lifetime τav of Curcumin in Ethanol; λ = 530 nma

a

T [K]

τav [ns]

T [K]

τav [ns]

312

0.22

210

0.89

296

0.26

197

1.00

285

0.30

185

1.08

273

0.33

165

0.94

260 247

0.40 0.45

148 124

1.01 1.11

235

0.52

86

1.34

222

0.67

τav =

R∞

0 Ifl

dt

temperatures. The main fitting parameters that vary with the temperature are the subvibration band peaks’ positions and their widths. Figure 1c shows the steady-state emission of curcumin in a 50% cyclohexane dioxane mixture. The fitting parameters have values similar to those of ethanol at low temperatures. There are four subbands with about the same positions and widths as in ethanol, though the relative amplitude of the first subband is larger than in ethanol. The steady-state measurements of Figure 1ac were collected by the CVI MS-240 diode-array spectrometer without further sensitivity corrections. Figure 2a shows the fluorescence up-conversion signals of curcumin in ethanol, measured at several wavelengths in the spectral range of 450610 nm. At λ e 510 nm, the average decay time of the signals is somewhat shorter than at longer wavelengths. The signals at longer wavelengths have a distinctive rise component followed by a relatively long decay time. Similar results have been recently obtained by Palit and co-workers.15,16 Figure 2b shows the fluorescence up-conversion signals of curcumin in 1-propanol. In 1-propanol, the rise and decay times of the signals are markedly longer than in ethanol. Figure 3 panels a and b show the constructed time-resolved spectra of curcumin in ethanol and 1-propanol, respectively. The spectra were constructed from the time-resolved emission data shown in Figure 2 by using a procedure similar to that described by Maroncelli and co-workers.17 As time progresses, the position of the peak of the band red-shifts, and the total band shift from

Figure 6. Time-resolved emission of curcumin in 1-propanol measured at several temperatures.

100 fs to 1 ns is ∼1800 cm1. Moreover, the red-shift of the first moments of the steady-state spectrum measured in 84 K with respect to the spectra measured at T g 250 K is roughly the same size as the total band shift from 100 fs to 1 ns in ethanol at room temperature. Figure 4 shows on a semilogarithmic plot the time-resolved emission of curcumin in ethanol measured at 530 nm, near the peak of the emission spectrum, at several temperatures in the range of 86312 K. The average decay time of the signal becomes longer as the temperature drops. The temperature dependence of the average decay time is strong at T g 235 K and much weaker below 235 K. The peak position and width of the steady-state emission spectra at T g 235 K (see Figure 1) are nearly invariant under a change of temperature. In the Discussion, we propose a model that explains these remarkable findings. The average decay values measured at 530 nm are given in Table 1. Figure 5 shows the TCSPC signals of curcumin in ethanol at 222 K measured at several wavelengths in the range of 470610 nm. As shown in the figure, at short wavelengths the decay is fast, whereas at λ g 530 nm the signal has a distinct rise followed by a relatively long decay time. The average decay time of the emission at 222 K is much slower than at room 10965



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temperature (see Figure 2). At a sufficiently low temperature, solvation is slow and comparable to the excited-state lifetime. The spectral width of the steady-state spectrum narrows, and the vibration structure appears. The shape and position of the spectrum are nearly temperature-independent at T < 165 K. We attribute the wavelength dependence of the time-resolved emission signals to solvation dynamics, as depicted by the constructed time-resolved spectra shown in Figure 3a,b. The solvation dynamics are much slower at low temperatures, as can be expected from viscous liquids such as ethanol and 1-propanol. Table 2. Average Lifetime τav of Curcumin in 1-Propanol; λ = 530 nma

a

T [K]

τav [ns]

T [K]

τav [ns]

338

0.31

197

1.18

325

0.34

185

1.22

312

0.38

173

1.20

296

0.38

160

1.19

286

0.47

148

1.18

273

0.53

136

1.18

260

0.63

124

1.17

247 235

0.75 0.86

112 100

1.17 1.16

222

1.01

86

1.19

210

1.12

τav =

R∞

0 Ifl

dt

Figure 6 shows the TCSPC emission signals of curcumin in 1-propanol at several temperatures measured at 530 nm. As in ethanol (see Figure 4), the average decay time becomes longer as the temperature decreases, and at any given temperature above 190 K, the decay time of the curcumin signal is shorter in ethanol than in 1-propanol. The average lifetimes of curcumin in 1propanol are given in Table 2. The Kinetic Isotope Effect. Figures 7 shows on a semilogarithmic scale the TCSPC signal of curcumin measured at 530 nm in ethanol and deuterated ethanol (CH3CH2OD) at several temperatures. Each panel shows the signals from the two samples at a certain temperature. At short times, the decay rates of both samples are the same, whereas at long times, the decay time of the signal from deuterated sample is substantially longer at all temperatures. The large difference in the long time component indicates that either a proton or a hydrogen atom is involved in the nonradiative decay of curcumin in the liquid state. We further elaborate on this important finding in the Discussion. Recently, an isotope effect on the average decay time of the curcumin fluorescence signal was observed in a study by Petrich and co-workers18 in both methanol and ethylene glycol samples at room temperature. The average lifetimes of curcumin in ethanol-d at several temperatures are given in Table 3. Main Findings a. A large blue spectral shift is observed in the steady-state emission spectrum as the temperature decreases in both ethanol and 1-propanol. Below 165 and 222 K for ethanol and 1-propanol, respectively, the emission band position and shape are nearly temperature-independent.

Figure 7. Comparison between the TCSPC signals of curcumin measured at 530 nm in ethanol and ethanol-d. 10966



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Table 3. Average Lifetime τav of Curcumin in Ethanol-d; λ = 530 nma

a

T [K]

τav [ns]

T [K]

τav [ns]

296

0.43

222

0.94

286 273

0.47 0.53

210 197

1.03 1.24

260

0.63

185

1.38

247

0.69

173

1.44

235

0.82

148

1.47

τav =

R∞ 0

Ifl dt

Scheme 2

Scheme 3

b. The decay time of the time-resolved emission signal is wavelength-dependent at intermediary and high temperatures in the range of 165330 K. The longer the detected wavelength, the longer the average lifetime of the signal. c. A distinctive isotope effect is observed on the long time decay component at all studied temperatures in the range of 84330 K.

’ DATA ANALYSIS AND DISCUSSION Qualitative Modeling of the Fluorescence of Curcumin. In this subsection, we propose that either intra- or intermolecular proton transfer enhances the nonradiative rate. This concept was also proposed in previous studies,15,18 and in this study we further develop it. In many previous studies of conjugated symmetric and asymmetric aromatic and heterocyclic compounds that are bridged by short linear hydrocarbon chains, it was revealed that the rotation of the ring systems at both ends of the bridge causes a rapid nonradiative decay process. We suggest that the nonradiative process curcumin undergoes is somewhat similar to those of auramine O (see Scheme 2) and thioflavin-T (ThT, see Scheme 3). Auramine O was extensively studied by Glasbeek and co-workers.20,21 They developed a photophysical model that explains the nonradiative process taking place in the electronically excited auramine O. They applied their model to explain the nonradiative process of auramine O.22 Auramine O is weakly fluorescent in low-viscosity solvents and fluorescent in viscous solvents. This type of fluorescence behavior is also found in ThT. Immediately after

a short pulse excitation, the initial population distribution resides in an excited emissive state (F, according to Glasbeek et al). It then diffuses by torsional motion toward a dark state denoted by D. The emissive state is coupled to the dark state as a function of the normalized twisting coordinate z. The adiabatic coupling of the two excited states is given by qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 ½FðzÞ DðzÞ2 þ 4C2 S1 ðzÞ ¼ ½FðzÞ þ DðzÞ 2 2 ð3Þ where F(z) and D(z) are the potential curves along the twisting coordinate of the emissive and dark diabatic states, respectively, and C is the coupling-strength parameter. Because the S1 state is a z-dependent mixture of fluorescent and nonfluorescent zeroorder states, the transition dipole moment, M(z), of the optical transition S1 f S0 is also z-dependent. The normalized S1 f S0 transition dipole moment decreases as a function of z: 2C 2 1 arctan MðzÞ ¼ cos ð4Þ 2 FðzÞ DðzÞ The transition dipole moment is small for the dark state and large for the emissive state, and therefore M(z) strongly depends on z. Meech and co-workers further studied auramine O and incorporated a time-dependent friction into the Glasbeek model.23,24 We recently suggested that the model proposed by Glasbeek and co-workers for auramine O also applies to ThT.25 As in auramine O, there are two excited states in ThT as well: The first is a locally excited state (LE), which is strongly emissive with a large dipole moment. The second is the result of free rotation of the rings about the C2C20 axis (see Scheme 3). This rotation about this axis couples adiabatically the emissive LE state with a charge transfer state (TICT state) of the aniline (dimethylaminophenyl) ring. The TICT state is characterized by a small transition dipole moment, and therefore it is considered a dark state. The plot of the ground-state potential surface versus the rotation angle, obtained by quantum calculations, has minima at ∼33 and 147° and a maximum at 90°, as opposed to the excitedstate potential surface, which has a broad shallow minimum at that angle, where it assumes a dark charge transfer state. In applying the principles of this model to curcumin, we have to consider a major difference between auramine O and ThT on one side and curcumin on the other, and that is that the former's average emission lifetimes are much shorter than that of the latter. The average lifetimes of ThT at the spectral emission peak position (490 nm) for methanol, ethanol, and 1-propanol are 3, 6, and 11 ps, respectively. For curcumin, those lifetimes are 120, 240, and 360 ps, respectively, which is roughly 40 times longer. Previously, we estimated the nonradiative rate of ThT in water by calculating the time it takes for aniline to rotate by ∼60°, which is the change in the C2C20 angle from the minimum of the ground-state potential to the minimum of the excited-state potential. This is also called the dark charge transfer. For that purpose, we qualitatively calculated the rotation time of a sphere in a viscous media of 1 cP. A sphere with a volume of 30 Å3, which is about the same as aniline, rotates by 60° in 1 ps. The ring system of curcumin has a slightly larger volume than dimethylaniline, but this argument alone cannot justify the fact that the nonradiative rate of ThT is 40 times as fast as that of curcumin. We therefore propose that the planar hexagonal ketoenol 10967



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hydrogen bonding subunit (see Scheme 1) inhibits the fast rotation of the two subunits by creating a high energy barrier for that process. In protic solvents, this hexagonal subsystem is perturbed by the formation of a hydrogen-bonded complex between the oxygen of a solvent molecule, the hydrogen of the enol, and the oxygen atom of the carbonyl with a second solvent molecule (see Scheme 1). In the excited-state, the curcumin assumes, at first, a planar hexagonal structure, and this first stage is emissive. The solvent-bridged hydrogen-bonded complex then breaks the flat hexagonal structure, and the aromatic moieties rotate by about 90° with respect to each other. This rotation creates the dark charge transfer state, which decays nonradiatively. The rate limiting step determining the nonradiative rate in protic solvents such as water or alcohol of any kind is the formation of the intermolecular hydrogen-bonded complex and breaking of the intramolecular hydrogen-bonded hexagonal planar ketoenol structure. This sequence of events also explains the moderate kinetic isotope effect (KIE) observed on the fluorescence lifetime of curcumin. Excited (intermolecular) proton transfer (ESPT) is a wellstudied process. Photoacids are usually hydroxyaromatic compounds that are weak acids in their ground-state, with pKa values ranging from 6 to 9 and much stronger acids in their excitedstate; that is, there is a large drop in the pKa value in the excitedstate, ΔpKa g 5. This attribute makes the photoacid a convenient tool for studying ESPT reactions. The ESPT rate depends on the pKa* value of the photoacid, in that the stronger the photoacid, the faster the rate and the larger the rate constant, kPT. Proton transfer occurs between a photoacid and a protic solvent molecule. Water is the protic solvent for which proton transfer is the fastest. For photoacids with a pKa* value of ∼0, the ESPT rate is ∼0.04 ps1, and in methanol it is slower by more than 2 orders of magnitude, i.e., 0.1 ns1. The KIE value is ∼3 for water, methanol, ethanol, and 1-propanol for moderately strong photoacids and decreases for strong photoacids dubbed “superphotoacids”. For superphotoacids with pKa* < 2, the KIE is usually smaller than 3. Temperature Dependence. In order to calculate the nonradiative rate constant, knr, from the experimental excited lifetime, we used the following procedure: a. The overall fluorescence-decay rate, k, is calculated from the time-resolved emission measured at 530 nm near the steadystate emission-band maximum. At shorter wavelengths, the average decay time is shorter and the deviation from exponential decay is large. At longer wavelengths, the timeresolved emission shows a build-up time component, which increases the average lifetime. b. We used the following relation to derive knr: k ¼ kr þ knr

ð5Þ

i knr ¼ ksol nr þ knr

ð6Þ

where k is the observed fluorescence-decay rate constant. ksol nr is the nonradiative rate relative to the solvent dielectric relaxation time. kinr includes all other nonradiative channels that are not related to the solvent dielectric relaxation. k ¼ kef f þ ksol nr

ð7Þ

where keff is the sum of the pure radiative constant, kr, and kinr, the nonradiative constant not related to the solvent viscosity. For the value of keff, we used the average decay

Figure 8. (a) Arrhenius plots of knr of curcumin in ethanol and ethanol-d solutions versus 1/T. (b) Comparison of the reciprocal of the dielectric relaxation of ethanol, 1/τD, with knr.

time of curcumin at the lowest temperature measured, τav ≈ 1.35 ns at 88 K. The pure radiative rate is smaller than keff, as can be calculated from the StricklerBerg equation, the molar extinction coefficient, and the absorption spectrum. Chignell et al. measured the fluorescence quantum yield, ΦF of curcumin in several solvents.13 They found for ethanol, ΦF = 0.063. The average emission lifetime for ethanol measured at 590 nm was found to be τav = 240 ps. From these two values, we deduce a pure radiative lifetime of 3.8 ns. We used a similar procedure to obtain ksol nr in ThT, for which the value of τav at T < 140 K was ∼3 ns in both ethanol and 1-propanol. This value is smaller than the expected value of the pure radiative lifetime of ThT. Nevertheless, the existence of other nonradiative mechanisms, which are not related to the main one, limits the effective radiative lifetime, τ0 r, of curcumin at low temperatures in very viscous solvents to 1.35 ns. Figure 8a shows the nonradiative rate constant ksol nr of curcumin in ethanol and ethanol-d as a function of 1/T. The activation energy of the nonradiative process in both ethanol and ethanol-d is relatively low, i.e., 10.4 ( 1.2 kJ/mol. At about 200 K, the 8 1 value of ksol nr is ∼2 10 s , and the overall decay rate constant is 8 1 ∼7 10 s . At temperatures below 200 K, the fluorescence decay rate is almost temperature-independent. We are unable to determine with reasonable certainty the value of ksol nr below 165 K because it is much smaller than the fluorescence decay rate constant. 10968



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Figure 10. Steady-state emission spectra of curcumin in methanol containing sodium acetate.

Figure 9. (a) Arrhenius plots of knr of curcumin in propanol versus 1/T. (b) Comparison of the reciprocal of the dielectric relaxation of propanol, 1/τD, with knr.

Figure 8b shows the nonradiative rate constant, ksol nr , of curcumin in neat ethanol along with the reciprocal of the dielectric relaxation time, 1/τD, of neat ethanol versus 1/T. At low temperatures, knr and 1/τD have similar values, following the same trend. Figure 9a shows the Arrhenius plot of ln ksol nr of curcumin in 1-propanol versus 1/T. The activation energy of the nonradiative process derived from this plot is 8 kJ/mol, which is almost twice higher than the activation energy of this process in ethanol. Figure 9b shows the nonradiative rate constant, ksol nr , of curcumin in 1-propanol, along with the reciprocal of the dielectric relaxation time, 1/τD, of 1-propanol versus 1/T. The nonradiative rate constant qualitatively obeys the same trend as the dielectric relaxation at all studied temperatures. Previously,26 we found similar correspondence between the ESPT rate constant of 5,8,-dicyano-2-naphthol (DCN2) and the dielectric relaxation time in methanol, ethanol, and 1-propanol. Tolbert et al.27 estimated that the pKa* of DCN2 is 4, making it one of the strongest photoacids in existence. As aforementioned, for photoacids as strong as DCN2 the kPT value is determined by the solvent reorganization time prior to the actual ESPT process. For “superphotoacids”, the solvent controls the ESPT rate to the solvent. The dielectric relaxation provides a decent estimate for solvent reorganization prior to the ESPT. Acetate Effect. As aforementioned, a plausible mechanism explaining the nonradiative decay process involves ESPT to the solvent. To uncover further evidence for ESPT, we also studied

ESPT from curcumin to a mild base introduced into the solution. The fluorescence lifetime of curcumin decreases in protic solvents such as methanol in the presence of acetate ions in concentrations higher than 0.2 M. The steady-state emission intensity decreases as the acetate concentration increases. Both measurements indicate that ESPT is involved in the nonradiative decay process of curcumin. The Pines and Nibbering research groups collaborated in order to extensively study the 8-hydroxy-1,3,6-pyrenetrisulfonate (HPTS)-Ac system.28,29 They used 400 nm femtosecond laser pulses to pump the HPTS to S1 and mid-IR femtosecond pulses to probe it. The IR probing allows for the monitoring of not only HPTS, for which visible probing is sufficient, but also the acetate and the H3O+. They found that, in addition to the long-time reaction ROH* + Ac f RO* + HAc, controlled by the diffusion process of both species, there are two kinds of ROH*Ac complexes reacting within the short-time regime. One kind of complex is a contact pair, where one of the oxygen atoms of the acetate is in close proximity to the hydrogen atom of the hydroxyl group of HPTS, and it reacts within 150 fs, which is the time resolution of the experimental system. The other kind of complex is a water-bridged complex, ROH 3 3 3 (H2O)n 3 3 3 Ac, for which the reaction rate is much slower and depends on the number of water molecules in the “solvent bridge”. Figures 10 and S1 (SI) show, respectively, un-normalized and normalized steady-state emission spectra of curcumin in neat methanol and in 0.2 and 1.7 M of sodium acetate (NaAc) in methanol. The position of the peak of the emission spectrum in the 1.7 M NaAc blue-shifts by about 13 nm (440 cm1). Smaller blue shifts were also observed in the emission spectra of several photoacids in aqueous acetate solutions. As seen in Figure 9b, the emission intensity of curcumin drops by a factor of 5 when 1.7 M acetate is introduced into the neat methanol solution. Figure 11 shows on a semilogarithmic scale the time-resolved emission of curcumin in neat methanol and in 1.7 M NaAc in methanol. As seen in the figures, the average decay-times of the TCSPC signals of curcumin measured at 530 nm in the presence of 1.7 M NaAc are considerably shorter than in neat methanol. Methanol was used as a solvent because it dissolves NaAc much better than ethanol or any other longer linear chain alcohol. Table S4 (SI) provides the average lifetimes of the TCSPC signals of curcumin in 1.7 M NaAc methanol solution. We attribute the large change in the steady-state emission intensity and the average lifetime of the TCSPC signals of 10969



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ferulic acid, feruloylmethane, vanillin, and acetone.31 Curcumin degradation in aqueous media at neutral pH is minimal.32,33 Another unanswered question is which hydroxyl group transfers a proton to the solvent. Because the curcumin is symmetric, both aromatic rings have one of their hydrogen atoms substituted by a hydroxyl group at the same position. Thus, ESPT can potentially occur from each of the three proton donors of the molecule, the enol at the center, or each of the phenols on the sides.

Figure 11. TCSPC curcumin signals measured at 530 nm in a methanol sodium acetate solution.

curcumin in 1.7 M NaAc in methanol to ESPT. According to the proposed model, the nonradiative rate accelerates when the planarity of the hexagonal ketoenol structure is broken. Proton transfer to the mild acetate base enables the rotation of the molecule’s two annular moieties with respect to each other. Having rotated, the molecule enters a charge transfer state, which has a very small transition dipole moment. Alternative Proton Transfer. We wish to propose an alternative explanation to the mechanism through which curcumin decays nonradiatively in an efficient manner. Ground-state curcumin is a weak acid with a pKa value of 7.5. If curcumin is a photoacid, then the excited-state pKa value will be much lower than its ground-state’s. As discussed in a previous section, the ESPT rate is correlated with the pKa* value. A strong photoacid has a low pKa* value, which translates to a faster ESPT rate constant. When pKa* e 2, the photoacid is considered strong, and this translates to a kPT value of g1011 s1 and an ESPT time constant of e10 ps. The ESPT rate from a photoacid to the solvent depends on both the properties of the photoacid and the solvent. It was found30 that for weak acids the pKa rises by more than two units when dissolved in methanol and rises even further when ethanol is used. Curcumin does not dissolve well in neat water, but it readily dissolves in watermethanol or waterethanol mixtures. The emission decay profile of curcumin in 80% aqueous solution of methanol is bimodal, as it has a major decay component at short times with a time constant of up to 2 ps followed by a minor component with a small amplitude at long times. The curcumin emission decay times in the three protic solvents, water, methanol, and ethanol, are similar, as was previously found in studies on strong photoacids. However, the curcumin spectrum lacks the emission of its conjugate base. When the curcumin is excited in its conjugate base form (basic solution), its fluorescence is very weak and its excited-state lifetime is only ∼50 ps. Figure S2 (SI) shows the absorption spectrum of curcumin in a basic solution of methanol at several wavelengths. Figure S3 (SI) shows the steady-state emission spectrum of curcumin in a basic methanol solution. This weak emission prevents the unequivocal identification of the curcumin’s photocycle as that which is characteristic of a photoacid. Figures S4 and S5 (SI) show the fluorescence up-conversion signals of curcumin in a basic solution of 102 M NaOH in ethanol measured on a wide range of wavelengths, 490640 nm. The basic solution degrades to

’ SUMMARY We used steady-state and time-resolved emission techniques to study the temperature dependence of the nonradiative decay process of curcumin in both ethanol and 1-propanol. In both solvents, the decay rate of the curcumin emission strongly depends on the temperature at T > 175 K. We calculated the dynamic part of the nonradiative rate by subtracting the fluorescence decay rate at T < 175 K, for which the decay rate is temperature-independent, from the fluorescence decay rate at any given temperature in the range of 175330 K. At the intermediary temperature range, 175250 K, ksol nr , the nonradiative rate constant related to the solvent viscosity, and the dielectric relaxation time qualitatively behave similarly. At low temperatures, T < 165 K, the steady-state emission spectra of curcumin in ethanol exhibit a subvibrational spectrum with spacing of about 1250 cm1. There is a red-shift of ∼2000 cm1 of the first moment of the curcumin emission spectrum at the intermediary temperature range of 175222 K. At higher temperatures, the shape and position of the spectrum are temperature-independent. We measured solvation dynamics in both ethanol and 1-propanol. As was indicated before by Palit and co-workers,15,16 the solvation dynamics are similar to that found in coumarin dyes, and the band red-shifts by about 1800 cm1 in ethanol. The value of the H/D isotope effect on the emission lifetime in ethanol is 2. We propose a model for the nonradiative process involving an ESPT step. There are three experimental observations indicating the involvement of ESPT in the nonradiative process. The emission lifetime is reduced by a factor of 4 in the presence of a mild base, 1.7 M sodium acetate in this case, in agreement with previous studies on ESPT from photoacids to mild bases. In ethanol-d, the lifetime is doubled at all studied temperatures. Petrich and co-workers18,19 also found an isotope effect on the lifetime of curcumin in methanol and ethylene glycol. The nonradiative decay rate behaves similarly to the dielectric relaxation time of the solvent at T < 250 K. The model is based on two molecular steps. The first, a proton is transferred from the planar and hexagonal structure of the ketoenol to the solvent. This enables the rotation of the two aromatic rings with respect to each other. At a dihedral angle of 90°, a charge transfer state with low oscillator strength is formed, and the emission is quenched. An alternative explanation to the fast nonradiative rate is the occurrence of ESPT, as it leads to the creation of an excited deprotonated form that simply has a fast nonradiative decay rate. Curcumin is a weak acid in the ground-state with pKa of ∼7.5. Excitation of the deprotonated form results in a very weak emission around 570 nm with a short lifetime of ∼10 ps in ethanol. ’ ASSOCIATED CONTENT

bS

Supporting Information. Steady-state and time-resolved emission data and data analysis, basic solution data and data

10970


The Journal of Physical Chemistry A analysis, and sodium acetate effect. This information is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: 972-3-6407012. Fax: 972-3-6407491.

’ ACKNOWLEDGMENT This work was supported by grants from the James-Franck German-Israeli Program in Laser-Matter Interaction and by the Israel Science Foundation. We would like to express our gratitude to Dr. Mordechai Erez for proposing this research topic and for providing us with preliminary insight on the photophysics of curcumin. ’ REFERENCES (1) Fisher, C. Phenolic Compounds in Spices, and Tønnesen, H. H. Chemistry of Curcumin and Curcuminoids. In Phenolic Compounds in Food and Their Effects on Health I. ACS Symposium Series 506; American Chemical Society: Washington, DC, 1992; Chapter 9, pp 118129, and Chapter 11, pp 143153. (2) Kita, T.; Imai, S.; Sawada, H.; Kumagai, H.; Seto, H. Biosci., Biotechnol., Biochem. 2008, 72, 1789–1798. (3) Kolev, T. M.; Velcheva, E. A.; Stamboliyska, B. A.; Spiteller, M. Int. J. Quantum Chem. 2005, 102, 1069–1079. (4) Payton, F.; Sandusky, P.; Alworth, W. L. J. Nat. Prod. 2007, 70, 143–146. (5) Hatcher, H.; Planalp, R.; Cho, J.; Torti, F. M.; Torti, S. V. Cell. Mol. Life Sci. 2008, 65, 1631–1652. (6) Jovanovic, S. V.; Steenken, S.; Boone, C. W.; Simic, M. G. J. Am. Chem. Soc. 1999, 121, 9677–9681. (7) Sharma, R. A.; Gescher, A. J.; Steward, W. P. Eur. J. Cancer 2005, 41, 1955–1968. (8) Bong, P. H. Bull. Korean Chem. Soc. 2000, 1, 81–86. (9) Priyadarsini, K. I. J. Photochem. Photobiol., C 2009, 10, 81–95. (10) Nardo, L.; Andreini, A.; Masson, M.; Haukvik, T.; Tønnesen, H. H. J. Fluoresc. 2011, 21, 627–635. (11) Nardo, L.; Andreini, A.; Tønnesen, H. H. In Hydrogen Bonding and Transfer in the Excited State; John Wiley & Sons: New York, 2010; Chapter 17, pp 353375. (12) Patra, D.; Barakat, C. Spectrochim. Acta, Part A 2011, 79, 1034–1041. (13) Chignell, C. F.; Bilski, P.; Reszka, K. J.; Motten, A. G.; Sik, R. H.; Dahl, T. A. Photochem. Photobiol. 1994, 59, 295–302. (14) Mukerjee, A.; Sorensen, T. J.; Ranjan, A. P.; Raut, S.; Gryczynski, I.; Vishwanatha, J. K.; Gryczynski, Z. J. Phys. Chem. B 2010, 114, 12679–12684. (15) Ghosh, R.; Mondal, J. A.; Palit, D. K. J. Phys. Chem. B 2010, 114, 12129–12143. (16) Khopde, S. M.; Priyadarsini, K. I.; Palit, D. K.; Mukherjee, T. Photochem. Photobiol. 2000, 72, 625–631. (17) Papazyan, A.; Maroncelli, M. J. Chem. Phys. 1995, 102, 2888–2919. (18) Kee, T. W.; Adhikary, R.; Carlson, P. J.; Mukherjee, P.; Petrich, J. W. Aust. J. Chem. 2011, 64, 23–30. (19) Adhikary, R.; Mukherjee, P.; Kee, T. W.; Petrich, J. W. J. Phys. Chem. B. 2009, 113, 5255–5261. (20) van der Meer, M. J.; Zhang, H.; Glasbeek, M. J. Chem. Phys. 2000, 112, 2878–2887. (21) Glasbeek, M.; Zhang, H. Chem. Rev. 2004, 104, 1929–1954. (22) Changenet, P.; Zhang, H.; van der Meer, M. J.; Glasbeek, M.; Plaza, P.; Martin, M. M. J. Phys. Chem. A 1998, 102, 6716–6721.

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Temperature Dependence of the Fluorescence Properties of Curcumin

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